Magneto-inductive, flow measuring devices utilize for volumetric flow measurement the principle of electrodynamic induction. When charge carriers of a medium move perpendicularly to a magnetic field, a measurement voltage is induced in measuring electrodes arranged essentially perpendicular to the flow direction of the medium and perpendicular to the direction of the magnetic field. The measurement voltage induced in the measuring electrodes is proportional to the flow velocity of the medium averaged over the cross section of the measuring tube, thus proportional to the volume flow rate. If the density of the medium is known, the mass flow in the pipeline, or in the measuring tube, can be determined. The measured voltage is usually tapped via a measuring electrode pair, which is arranged, as regards the coordinate along the measuring tube axis, in the region of maximum magnetic field strength and where, thus, the maximum measurement voltage is to be expected. The measuring electrodes are usually galvanically coupled with the medium. There are, however, also magneto-inductive, flow measuring devices with capacitively coupled, measuring electrodes. The magnetic field is, most often, periodically reversed, so that measurement voltages with reversing sign arise on the measuring electrodes alternatingly. Besides the measuring electrodes, a magneto-inductive, flow measuring device can also have measured material monitoring electrodes for detecting partially filled or empty measuring tubes and/or reference, or grounding, electrodes for the electrical reference potential between measuring device and measured material.
Usually, the voltage signals of the electrodes of a magneto-inductive, flow measuring device are fed to a differentially working amplifier, called a differential amplifier, for short. This amplifies the difference of the two voltage signals of the electrodes with an amplification gain G. The output signal of the differential amplifier is supplementally increased by an amplifier referenced, offset signal and is then fed to an analog to digital converter, which is referred to in the following as the A/D converter. The voltages are referenced, in such case, to a certain, fixed reference potential, such as e.g. ground, or to a reference electrode of the magneto-inductive, flow measuring device, with which both differential amplifier as well as also the A/D converter work
German Patent, DE19716151C1, describes the production of a reference potential. For this, the differential amplifier is connected with a reference electrode or with a measuring electrode.
The wanted signals of the two electrodes are very small in comparison to superimposed, disturbance signals, which are e.g. common-mode signals. The wanted signals lie, conventionally, in the region of a few μV, while the disturbance signals can amount to a few V. Thus, either a high quality of the A/D converter, especially as regards its noise and/or its resolution, is necessary, in order to be able to further process the wanted signals as well as possible, or a suppressing, or filtering out, of the disturbance signals and subsequent amplification of the remaining, wanted signals is used. A/D converters with high resolution are comparatively expensive.
German Patent DE19906004A1 discloses a suppressing, or filtering out, of a common-mode signal by a suppressing of low frequency fractions in the difference signal. In this regard, a preamplifier represents a highpass filter, whose practical implementation leads, however, to a lack of symmetry between the signal paths. For reducing this problem, a resistor network is described. Through the suppressing of the low frequency fractions in the difference signal with the assistance of the highpass filter, thereafter, a high amplification is possible. The amplified signals are then fed to an A/D converter having differential inputs.